RTC Expressions : How to create a dynamic expression to get a particular Team's Unassigned workitems ?

Hello All,

I have a usecase wherein, I need to create a RTC Expression for fetching all the "Unassigned workitems" for a given Team Area.

I am following : https://rsjazz.wordpress.com/2012/11/19/using-expressions-for-automation/

How I didnt a insight of using the "TeamArea" and its planned for "Unassigned" in the expression.

I tried with following  :
 
                IQueryableAttribute teamAttribute = QueryableAttributes.getFactory(ITeamArea.ITEM_TYPE)
                    .findAttribute(projectArea, ITeamArea.NAME_PROPERTY_ID, auditableCommon, monitor);

Is this correct ?
            
but not getting any clue for the "Planned for" : Unassigned workitems.

Please help me forming this query.

Thanks